1. Field of the Invention
This invention relates to an apparatus for controlling an air flow of a boiler by controlling an air flow by at least one of a first fan for feeding air to the boiler and a second fan for sucking air from the boiler.
2. Description of the Prior Art
A fan for forcing air into a boiler or for extracting air from a boiler is driven by a variable speed electric motor in order to allow adjustment of an air flow. A pole change motor (hereinafter referred to as "PAM motor") is widely employed as a variable speed electric motor which is suitable for such applications.
FIGS. 1(a) and 1(b) are diagrammatic representations illustrating a principle of a pole change motor (PAM motor), and in those figures, reference symbols 1a, 1b, 2a, 2b, 3a, 3b, 4a, and 4b designate each a stator winding (each modelled for one phase), and reference symbol 5 designates a pole (indicated by N or S) of a rotating magnetic field.
FIG. 2 is a circuit diagram illustrating a principle of a pole change system for a PAM motor, and in this figure, reference symbol 6 designates stator windings including windings 61a, 61b, 62a, 62b, 63a and 63b and having terminals U.sub.1, U.sub.2, V.sub.1, W.sub.2, W.sub.1 and V.sub.2, respectively. Reference symbols 7, 8 and 9 denote each a switch, VR, VS and VT denote power supply voltages of R, S and T phases of a three-phase power source, respectively, and reference symbol o designates a neutral point of the three-phase voltages. Referring to FIG. 3, reference symbol 10 designates an electric motor, 11 a ventilator, 12 a shaft which interconnects the electric motor 10 and the ventilator (fan) 11, and 14 an air course resistance controlling mechanism including a bar 14a which is moved up and down to move a damper 14b accordingly to increase or decrease a resistance of an air course. Reference symbol 13 denotes a controlling signal which indicates an opening of the damper 14b necessary for the air flow resistance controlling mechanism 14, and reference symbols 15 and 16 designate an entrance and an exit of the air course, respectively.
It is to be noted that FIG. 1(a) is a diagrammatic representation, in a modelled form, of a pole change motor having four poles therein while the motor is used as an electric motor having six poles therein by reversing the polarity of electric currents flowing through the coils 2b, 3a, 3b and 4a which are shown in broken lines in FIG. 1(b). In this way, a PAM motor having variable poles therein can be obtained by changing connections of some of its stator windings to change coil currents. While FIG. 1(a) and 1(b) illustrate an example wherein the polarity of electric currents is changed, the polarity may otherwise be changed by changing phase currents.
Further, it is to be mentioned that the electric motor in FIG. 2 is run at a low speed with the switch 7 closed and with the switches 8 and 9 open, and on the contrary, it is run at a high speed with the switch 7 open and with the switches 8 and 9 closed to change the electric currents flowing through the stator windings 6 to change the number of poles of the motor.
Correspondence of the change of the number of poles between FIG. 1 and FIG. 2 will be described below. In particular, if description is given by way of an example of an electric current of the R phase, the coil 61b of FIG. 2 is connected between the terminal U.sub.2 and the neutral point o and the direction of the electric current flowing therethrough does not change after changing of the number of poles. Accordingly, the coil 61b corresponds to the coil 1a, 1b, 2a or 4b of FIGS. 1(a) and 1(b). On the other hand, the coil 61a is connected between the terminals U.sub.1 and U.sub.2 and the direction of the electric current flowing therethrough changes after changing of the number of poles. Accordingly, the coil 61a corresponds to the coil 2b, 3a, 3b or 4a of FIGS. 1(a) and 1(b). Since the rotational frequency n of an electric motor is given by a following expression, ##EQU1## f: power source frequency [Hz] p: number of poles
the rotational frequency can be varied by changing the number of poles of the motor. A load to the electric motor may sometimes vary. For example, a boiler forcing fan connected to an electric motor may run under a full load in the daytime and under a low load at night. Frequently, from a point of view of saving power at night, the electric motor may be run in a lower rotational frequency (with an increased number of poles) in accordance with a low load, and in the daytime, it may be run in an increased rotational frequency (with a reduced number of poles) in accordance with a heavy load.
Thus, the rotational frequency of the PAM motor is changed by changing over of the switches 7, 8 and 9 of FIG. 2. Rotation of the PAM motor is transmitted to the ventilator (fan) 11 through a rotor of the motor 10 and the shaft 12. In this instance, a signal representative of a deviation of an actual air flow at present from a required air flow is delivered as a controlling signal 13 to the air course resistance controlling mechanism 14. The bar 14a is moved up or down in response to the controlling signal 13 to move the damper 14b accordingly to control the air flow.
Since a conventional air flow controlling apparatus is constructed as described above, when a PAM motor is to be changed over from a high speed to a low speed running or vice versa, harmonization between variation of an air flow due to variation of the rotational frequency and variation of an air flow by means of the damper 14b cannot be attained in a transient state in which the rotational frequency of the PAM motor varies. Accordingly, where a load to the fan is a boiler or the like, there may be a risk that a fire of the boiler during combustion goes out, a change of an internal pressure reaches a limit explosion of the boiler, and so on. Thus, the conventional air flow controlling apparatus is disadvantageous in that a PAM motor cannot be applied thereto.
In the meantime, an air flow controlling apparatus has also been put into practice wherein a motor which is driven from a variable frequency power source is employed as a motor for driving a fan in order to accomplish regulation of an air flow of the boiler as described above. In this case, the motor 10 receives supply of power alternatively from a variable frequency power source 24 (hereinafter referred to as "V power source") or a commercial power source 25 (hereinafter referred to as "C power source") depending upon open and closed conditions of the switches 21, 22 and 23. FIG. 5 illustrates operating characteristics when power supply is changed over to the C power source 25 because of a trouble of the V power source 24, and in this figure, reference symbols t.sub.1, t.sub.2 and t.sub.3 designate a point of time at which the trouble has occurred to the V power source 24, another point of time at which the C power source 25 is coupled, and a further point of time at which rotation of the motor reaches a particular rotational frequency determined in accordance with the frequency of the C power source, respectively.
The conventional boiler air flow controlling apparatus is constructed as described above. Accordingly, the conventional boiler air flow controlling apparatus is disadvantageous in that, if a trouble occurs to the V power source and thus supply of power is changed over to the C power source, the rotational frequency of the motor (and hence the first fan 11) rises suddenly to increase an air flow while controlling of an air flow by means of the damper 14b is slow in responsiveness so that a wind pressure within the boiler increases to deteriorate safe running of the boiler, resulting in the necessity of tripping of the boiler. Such circumstances will be described with reference to FIG. 5.
If a trouble occurs to the V power source (at the time t.sub.1), then the rotational frequency N.sub.1 of the motor (the fan) decreases and the air flow Q.sub.1 by the first fan 1 decreases accordingly. At the time t.sub.2 after lapse of a predetermined period of time, the motor is energized by the C power source so that the rotational frequency N.sub.1 of the motor rises and the air flow Q.sub.1 rises accordingly. If the inner pressure of the boiler rises higher than a predetermined level (for example, 200 mmHg), it is a dangerous range, which is in indicated by T in FIG. 5.
Accordingly, the conventional boiler air flow controlling apparatus having the construction as shown in FIG. 4 has a defect that the boiler must be tripped (stopped) at a point of time when such a dangerous range is entered.